The ethylene cracking techniques used in petrochemical ethylene equipments mainly include those developed by LUMMUS Co. (USA), Stone & Webster Co. (USA), Kellog & Braun Root Co. (USA), Linde Co. (Germany), Technip KTI Co. (Netherlands), and the CBL cracking furnace developed by China Petrochemical Corporation.
FIG. 1A shows a typical ethylene cracking furnace 10, which comprises a radiant section 11, a convective section 13, and a flue section 12 located between the radiant section 11 and the convective section 13. Within the radiant section 11 a set of multi-pass radiant coil 14 is provided in the central plane P of the radiant section 11 along the longitudinal direction thereof In addition, the radiant section 11 is further provided with bottom burners 15 and/or side burners 16 for heating. Moreover, the ethylene cracking furnace 10 further comprises a transfer line exchanger 17, a high-pressure steam drum 18 and an induced draft fan 19, etc. Nowadays, in most companies a four- to six-pass (˜60 meter) medium-selectivity radiant coil with or without branches of variable diameters is used and the residence time thereof is controlled within the range of 0.4 to 1.0 second, so that the radiant coil suitable for cracking gas material can be also adapted for liquid material, with a proper operation cycle and a satisfied material adaptability. The first tube or the first two tubes of the radiant coil are of small diameter. Therefore, a quick temperature rise can be achieved since the specific surface area of the small-diameter tubes is relatively large. The tubes following the second tube are of large diameter, in order to reduce the influences on coking sensitivity. The four-pass medium-selectivity radiant coil used can be configured as 4-2-1-1 type, 2-2-1-1 type, 1-1-1-1 type and 2-1-1-1 type, etc.
For the radiant coil 14 of the prior arts, all tubes thereof are generally spatially arranged in sequence along the flow direction of fluid. In the meantime, the tubes are connected to each other with common elbows.
FIG. 1B shows a typical arrangement of multi-pass radiant coil of an ethylene cracking furnace in the prior art. As shown in FIG. 1B, the multi-pass radiant coil 30 is a four-pass radiant coil, comprising the first tube 1, the second tube 2, the third tube 3 and the fourth tube 4 along the fluid flow direction (i.e., from left to right in the drawing). The first tube 1 is a Y-shaped branched tube with varied diameters. All the four tubes are spatially arranged in sequence, i.e., the second tube 2 being arranged spatially between the first tube 1 and the third tube 2, and the third tube 3 being arranged spatially between the second tube 2 and the fourth tube 4, and so on. That is to say, each tube is arranged spatially adjacent to the one or two tubes that are consecutive to said tube. In addition, the tubes are connected to each other by means of common elbows 35. FIG. 1B further shows all tubes and the elbows are located in one single plane, i.e., the central plane P of the radiant section.
In this typical arrangement, the tubes are spatially arranged in sequence. Since the fluid flows along the multi-pass radiant coil from the first tube to the second tube, then to the third tube and finally to the fourth tube, the tube wall temperature of these tubes is gradually increased in this order. In other words, the temperature of the first, second, third and fourth tubes is gradually increased in this order. Therefore, an uneven temperature distribution is generated in the radiant section. Moreover, radiant heat exchange will also exist between the high-temperature tubes, i.e., the third and the fourth tubes in this case, which would negatively influence on reducing the tube wall temperature and extending the operation cycle of the furnace.
Furthermore, in the multi-pass radiant coil of the prior arts, the tubes are connected to each other by means of common elbows. This is undesirable for absorption of heat expansion generated in each of these tubes. In a long time operation, it will easily lead to bending of tubes, lowering the lifetime of the radiant coil and therefore shortening the operation cycle thereof.